Because electronics can implement complex functions relatively easily, security systems with electronic locks and keys can provide the high degree of security required in many situations. For example, as in the electronic code controlled deadbolt disclosed by Kristy in U.S. Pat. No. 4,568,998, a radio transmitter can selectively transmit coded "closed" or "open" signals. A receiver, including close and open decoder means and logic means separately responsive to these decoder means can produce outputs which actuate a switch to control a motor. The motor drives the deadbolt between the closed and open positions.
In the security system disclosed by Bosnyak, et al, in U.S. Pat. No. 3,761,892, permutations of a number system are used to create addresses for read-only memories (ROMs). A paired electronic lock and key each contain identical copies of such a ROM. The address code is transmitted from the lock to the key and, in response, the contents of the addressed location of the ROM are retransmitted from the key back to the lock. An identical comparison of the transmission received by the lock with the addressed contents of the lock's ROM signifies that a key associated with that electronic lock is requesting access to the area secured by the electronic lock.
Hardware-based systems such as those discussed above are relatively easily defeated, either by recording the signals exchanged by the lock and key to dissect the code or by copying the read-only memory.
On a more sophisticated level, security systems can be designed to generate a new key code whenever desired. For example, as disclosed by Donath et al., in U.S. Pat. No. 4,209,782, a "central" key can be used to generate a random number which is stored in a memory in the electronic lock and transferred to an electronic key, which uses the number as the next-used security code.
Stellberger, in U.S. Pat. No. 4,509,093, discloses a security system whose electronic lock, upon excitation by a signal received from the electronic key, creates a random number which is transmitted back to the electronic key. This number is transmitted to the key and subjected to the same two-step computational process in both key and lock. The computational result from the key is transmitted back to the lock, wherein it is compared with the results of the computations in the electronic lock itself. If the two results are identical, an actuation pulse is transmitted to unlock the gate being secured. Otherwise, the gate remains locked.
Stamm, in U.S. Pat. No. 4,353,064 discloses an access control card for use with a remote card reader. The card reader transmits coded radio frequency signals, the transmitted code being compared to a code stored in a memory of each operating card that receives the signals. If the received and stored codes are identical, the card transmits a signal coded with a second code stored in the card's memory. If the card reader recognizes the transmitted second code, access is given by the card reader. However, no provision is made in Stamm's card for different second code numbers to be used by separate cards. This feature is necessary where it is desirable to allow differing levels of access to different cards. Furthermore, all cards receiving the signal transmitted by the card reader will retransmit their responses at the same time. The resulting confusion of responses can lead to inaction or faulty operation by the card reader.
The problem of overlapping responses from a number of tags is approached by Barrett, Jr., et al, in U.S. Pat. No. 4,471,345. This patent discloses a portal communication system for monitoring the passage of tags past a portal location. The identification tags generate responses to an interrogation signal sent by the portal. These responses are randomly delayed in order to reduce the probability that two response signals overlap. However, the system disclosed requires that each tag must have a distinct identifying code. This complicates the processing required if the task at hand is to monitor tags worn by persons who are members of broad categories that are to be monitored (e.g., doctors, or nurses, in a hospital).
For increased security and to allow a variety of levels of access, it would be advantageous to have a security system with one or more access controllers and one or more associated electronic keys, each access controller having a predetermined, possibly unique, controller code and each electronic lock having its own, possibly unique, key code. In addition, such a security system will have improved performance if each key is given its own response time delay. It would further be advantageous for the security system to encipher the transmitted signals with random numbers, in order to provide even greater security. Finally, it would be advantageous to have a security system whose electronic keys conserve electrical power.